1. Field of the Invention
The present invention pertains to valves. Specifically, it pertains to a relief valve suitable for relieving excessive fluid pressures in a fluid pressure system.
2. Brief Description of the Prior Art
Valves are used in a variety of applications and designs. One application is for relieving excessive pressures in pressure vessels or fluid systems. Such valves are sometimes called relief valves. The purpose of a relief valve is to relieve excessive pressures in the vessel or pressure system to which it is attached to avoid damage to the vessel or system and to prevent hazardous situations. There are, of course, many types of relief valves. Some such valves open at preset pressures and are automatically reset or closed when pressure is reduced to a certain level. In other relief valves manual resetting is required.
A variety of relief valves are used in the oil and gas industry. One particular application in the oil and gas industry is for relieving excessive pressures in drilling fluid systems. The fluid being handled in a drilling fluid system is usually abrasive, may be corrosive and is very harsh on a valve. Due to the high risk to life and property, such a valve must be extremely reliable, able to function under adverse conditions and essentially tamper-proof in the field. Some of the most popular pressure relief valves suitable for use with oil and gas drilling fluid systems are those manufactured by OTECO Equipment Company of Houston, Tex.; DEMCO of Tulsa, Okla.; and Cameron Iron Works, Inc., of Houston, Tex. Several such valves utilize a plug or piston type closure member which is pinned in place by a nail or shear pin. When pressure in the system exceeds a certain value, the nail or pin is sheared and the pressure relieved. However, nails and pins are not always of the same strength. Furthermore, due to the fluctuating pressures in such a system, the nail or pin material may be fatigued so that it may shear at a lower pressure than desired. When this occurs, many workers insert a larger nail or pin or in some cases, a member of much greater shear strength such as an Allen wrench. As a consequence, the valve may not open at the desired pressure level or in fact may never open at all, causing serious danger. In addition such a valve must be manually reset after it is opened.
Other relief valves manufactured and sold by Cameron Iron Works, Inc. of Houston, Tex., and Weatherford/DMC of Houston, Tex., utilize spring-loaded valves. Some of these valves are manually reset and others are automatically reset. However, they are relatively complex and expensive to manufacture and maintain. Furthermore, accurate field adjustment of such valves is not always possible.
One of the problems associated with a relief valve of the type to be utilized with drilling fluid or mud systems is erosion. If the valve only gradually opens or quickly cycles between an open position and a closed position, the valve is likely to be severely damaged or destroyed by erosion. Thus, it is desirable for such a valve, when it opens, to quickly move to a full open position so that such erosion does not occur. This suggests the use of a pilot system or detent mechanism by which the valve closure member is not actuated until there is sufficient force to assure a full opening. Various types of pilot or control systems for opening of valves may be seen in U.S. Pat. Nos. 2,736,297; 3,132,668; 4,026,326; and 4,073,466. Such systems are relatively complex and expensive. A common form of detent mechanism utilized in such valves is a metal ball located in a hole at the surface of the member that is movable with respect to an adjacent member. When the adjacent member moves in a direction that lines up the ball with a detent, the ball is then forced into the detent by gravity, a spring or other means. The depth of the detent is about equal to the ball radius so that the ball is at the intersection of both adjacent surfaces. In this position, the surfaces cannot move with respect to each other.
To increase the holding power of such detent mechanism, a plurality of balls in a groove may be used. However, since balls have point contact, their load-bearing capacity without exceeding the yield point is relatively low. This may result in galling or damaging of the balls and malfunctioning of the mechanisms.